Ways to Measure Strain

Contacting

Strain measurement in materials testing is traditionally carried out using some form of contacting extensometer. A typical clip-on extensometer, for example, attaches to the specimen with clips or elastic bands and uses knife-edges to accurately track deformation in a specimen during testing.

While providing accurate strain measurement in numerous applications, contacting extensometers carry some inherent disadvantages; and therefore,it is highly recommended that the user review the section on "Considerations when Choosing an Extensometer" and carefully consider all aspects of the testing application before selecting the proper device.

Automatic Contacting

The need for increased specimen throughput has created a demand for faster, more reliable, and more accurate materials testing routines. Automatic Contacting Extensometers provide this capability by enhancing the productivity of testing laboratories that can take advantage of the automatic gauge length positioning feature, as well as the automatic attachment to the test specimen.

Suitable For

Almost all materials and a variety of geometries, such as, strip, round, hexagonal and flat shape.

Overview

Travel

GL: > 10 mm
750 mm

Accuracy

★ ★ ★ ★ ★

Temperature Range

Ambient (10 to 38 °C)

Automatic or Manual

Automatic

Common Standards

Averaging Axial and Biaxial

Averaging Axial and Biaxial Extensometers are used for testing advanced composites, rigid plastics and metals. Average axial strain is determined from independent strain measurements on opposite sides of the specimen. The use of average strain eliminates errors due to specimen bending and allows the determination of precise modulus values. By integrating a transverse strain transducer the extensometer can simultaneously measure average axial and transverse strain allowing the determination of Poisson’s ratio. Generally, this type of extensometer is intended for measuring small strains such as those used for the determination of elastic properties.

Suitable For

Dynamic clip-on extensometers are used for cyclic strain measurement in tension, compression or reverse-stress through zero measurements on a wide range of materials and components including metals, plastics, and composites.

Overview

Long Travel

The long-travel (XL) extensometer is a precise device for measuring strain in highly extensible materials . Designed for use with electromechanical testing instuments, long-travel extensometers can be used to measure specimens elongation's up to 10 in or 250 mm. The XL is designed to clamp directly onto to a specimen quickly and easily.
It will adjust to incremental gauge lengths ranging from 0.5 in to 5 in or 10 mm to 200 mm.

Suitable For

Elastomers, semi-rigid plastics, and films

Overview

Travel

GL: 10 mm – 100 mm
300 mm or 750 mm Maximum Travel

Accuracy

★ ★ ☆ ☆ ☆

Temperature Range

Ambient

Automatic or Manual

Manual

Common Standards

Static Clip-On

Clip-on extensometers for static testing offer speed of attachment and ease of use. Their light-weight, rugged cross brace design eliminates errors caused by physical distortion, while built-in protection ensures that damage is not caused by over-extension. The low operating-force arms of the extensometer reduce the possibility of knife-edge slippage when testing hard or smooth surfaced materials.

Suitable For

Static clip-on extensometers are used for static strain measurement on a wide range of materials and specimens including metals, plastics, and composites.

Overview

Travel

Wide variety of selections

Accuracy

★ ★ ★ ★ ☆

Temperature Range

-100 to 200 deg. C

Automatic or Manual

Manual

Common Standards

Transverse

Transverse clip-on extensometers are used to measure transverse (or width) strain. Versions with small travels (< 0.5 mm) are used for the determination of elastic properties versions with larger travels (> 5 mm) are suitable for the determination of plastic properties.

Suitable For

Static tests to determine r-value (plastic strain ratio) on sheet metal, Poisson’s ratio on composites, plastics and metals.
NOTE: An axial extensometer is also required for these tests

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Non Contacting

Several methods of non-contacting strain measurement exist, ranging from the most basic method of measuring displacement of the crosshead to full-field strain mapping using Digital Image Correlation (DIC). It is highly recommended that the user carefully consider all aspects of the testing application and review the section on "Considerations when Choosing an Extensometer" before selecting the proper device.

Video

A video extensometer is a non-contacting extensometer that can measure deformation by tracking the movement of two attached markers on the specimen, using high-resolution digital camera technology. Key advantages of video extensometers are that they do not exhibit any influence on the test specimen, problems with knife-edge slippage are eliminated, and errors due to inertia of moving parts is also eliminated.

Suitable For

Almost every material

Overview

Travel

GL: > 5, 10 mm, 60 mm, 200 mm, 500 mm

Accuracy

★ ★ ★ ★ ☆

Temperature Range

Can be used with Chamber and Bath

Automatic or Manual

Automatic

Common Standards

Conforms to the most rigorous testing standards, including ISO 527 and ASTM D638

DIC

Digital Image Correlation (DIC) is a strain measurement technique that works by capturing a series of images throughout a test and analyzing them afterwards. A system that includes a camera system, lighting, and a software package to control image capture and to conduct post-test analysis. Recently integrated DIC systems, which use existing video extensometers, have been introduced to provide a streamlined package tailored for the materials testing market.

Crosshead Displacement

Although crosshead extension can be an acceptable and common method of measuring strain, it is often an inadequate way as it will record the deformation of the entire specimen between the grips instead of just the narrow, parallel section which is the area of interest (the gauge length).

Bonded Strain Gauges

Bonded strain gauges are small metallic circuits which are glued to the surface of a specimen. As the circuit deforms the resistance changes, and this change in resistance is then turned into a strain recording. At the end of the test the bonded strain gauge is thrown out at a cost of around $5 per test. Bonded strain gauges are very accurate but are limited to strains under 10%. They take a highly skilled operator several minutes to apply, and require using curing ovens for hours at a time to set the adhesive.

Suitable For

Composites, component, and metals testing

Overview

Travel

★ ☆ ☆ ☆ ☆

Accuracy

★ ★ ★ ★ ★

Temperature Range

-80 to 250 deg C

Automatic or Manual

Manual

Common Standards

Learn more about...

Special Use

Deflectometer

The Deflectometer on its own is not a strain measuring device – it is simply a spring loaded plunger that is used in conjunction with an extensometer, whether it be a clip-on, an automatic contacting extensometer (AutoX750), or a video extensometer (AVE2). It is primarily used for flexural tests.

LVDT

Linear Variable Differential Transformer (LVDT) displacement transducers are measuring devices that measure the displacement of the load frames of materials testing systems. They feature an LVDT as the active element, and are designed for use on both electromechanical and hydraulic Instron® testing systems.

Suitable For

Almost all materials – dependent on travel

Overview

Accuracy

★ ★ ★ ☆ ☆

Temperature Range

-40 to +100 °C

Automatic or Manual

Manual

Common Standards

High-Temperature Uncooled Clip Ons

Most clip-on contacting extensometers use strain gauges to measure strain on the specimen. These devices use a high-temperature capacitive sensor allowing them to be used up to maximum temperature of most environmental chambers used in materials testing.

Overview

High-Temperature Uncooled Clip Ons

Most clip-on contacting extensometers use strain gauges to measure strain on the specimen. These devices use a high-temperature capacitive sensor allowing them to be used up to maximum temperature of most environmental chambers used in materials testing.